The present invention relates to process variable transmitters. In particular, the present invention relates to terminal blocks in process variable transmitters.
In processing plants, gases and liquids are routed through various conduits and tanks. In order to control the operation of a plant, the current status of the liquids and gases in the various portions of the plant as well as the state of controlled devices such as valves and burners must be monitored. The variables monitored to determine these states are referred to generally as process variables and can include flow rates, pressures, differential pressures, temperatures, tank levels, valve positions, and the like. These process variables are provided to a control room by field devices known as process variable transmitters. Each process variable transmitter includes a transmitter housing that encases at least one circuitry module used to determine a value for a process variable. The circuitry module determines the value for the process variable based on one or more sensor signals it receives from one or more process variable sensors that are measuring a state of the process fluid or a controlled device. The circuitry module also transmits the value of the process variable to a control room using either wireless communication or wired communication. In some cases, the circuitry module transmits the process variable using a two-wire process control loop which is also used to power the process variable transmitter.
In some process variable transmitters, the sensors are connected to the circuitry module by wires that run from the sensor to sensor line connections on a terminal block in the transmitter housing. These terminal blocks may also include power line connectors for connecting power lines to the process variable transmitter.
Because the terminal block receives sensor lines that are connected to the sensors, there is a chance that process fluid will enter the transmitter housing if the process fluid breaches the sensor or a vessel holding the sensor. To avoid damage to the circuitry module and to reduce the risk of the circuitry module igniting the process fluid during such a breach, many process variable transmitters have two chambers that are sealed off from one another by a sealing layer. The first chamber houses the circuitry module and the second chamber houses the terminal block. Fixed conductors attached to the connectors of the terminal block extend through the sealing layers between the chambers and attach to the circuitry module. The permanent nature of the electrical connections between the fixed conductors and the connectors of the terminal block help to ensure that the sensor signals are not corrupted by poor connections between the terminal block and the circuitry module. Such poor connections can negatively impact the sensor signal and result in faulty process variable values.